1. Field
The disclosure relates to an electric power conversion system that is able to step up or step down voltage for two direct-current power supplies in parallel with each other.
2. Description of Related Art
In a hybrid vehicle or an electric vehicle, which uses a rotary electric machine as a driving source, the rotary electric machine is driven by alternating-current power that is converted by an inverter from the direct-current power of a battery. In addition, a step-up/step-down converter is provided between the battery and the inverter. The step-up/step-down converter steps up a battery voltage or steps down electric power regenerated by the rotary electric machine.
A voltage converter is, for example, described in Japanese Patent Application Publication No. 2014-193090 (JP 2014-193090 A) as the one that has the extended function of the step-up/step-down converter. The voltage converter includes four switching elements, and is connected to two batteries. The voltage converter is able to switch the two batteries between series connection and parallel connection.
The above-described voltage converter steps up or steps down voltage for the two batteries in parallel with each other at the time of the parallel connection (parallel mode). Step-up/step-down operation is controlled via a PWM signal indicating a duty ratio to each of step-up/step-down circuits. The duty ratio is the ratio of an on time to a single PWM control period. The voltage converter described in JP 2014-193090 A has such a circuit configuration that the switching elements are shared between two step-up/step-down circuits. The on/off operation of each switching element is controlled in accordance with the logical addition of both PWM signals based on so-called principle of superposition. For example, when a predetermined one of the switching elements is controlled based on the PWM signal PWM1 for one of the step-up/step-down circuits and the PWM signal PWM2 for the other one of the step-up/step-down circuits, the on/off operation of the predetermined one of the switching elements is controlled by a composite signal of the PWM1 and the PWM2.
A power loss (hereinafter, also simply referred to as loss) arises with the on/off operation of each switching element. Specifically, examples of the loss include a switching loss (turn-on loss) that arises at the time when each switching element switches from an off state (interruption of current) to an on state (conduction of current) and a switching loss (turn-off loss) that arises at the time when each switching element switches from the on state to the off state, as shown in the upper timing chart of FIG. 17. Another example of the loss includes a steady loss that arises due to an on voltage (collector-to-emitter saturation voltage) at the time when each switching element is in the on state and a current flowing at this time.
The steady loss is classified into an overlap loss and an on-state loss. The overlap loss is caused when currents from the two step-up/step-down circuits are overlappingly supplied to each switching element. The on-state loss is caused when only current of one of the two step-up/step-down circuits is supplied to each switching element. Because of the magnitude relation in current, the overlap loss is larger than the on-state loss.
Each switching element is heated by the loss that arises in the switching element. In order to prevent overheating of each switching element, the phases of the PWM signals for the two step-up/step-down circuits are shifted from each other (phase shift control) in JP 2014-193090 A.
In phase shift control, as shown in the lower timing chart of FIG. 17, one or both of the phases of the PWM signals PWM1, PWM2 are shifted such that the leading edge of an on duty (OnDuty1) of the PWM signal PWM1 is brought into coincidence (connection) with the trailing edge of an on duty (OnDuty2) of the PWM signal PWM2. Thus, the number of times of switching is reduced as compared to the PWM signals shown in the upper timing chart of FIG. 17, with the result that the switching loss is reduced. In addition, the duration of the overlap loss is also shortened.